The electrophoretic effect is well known and the prior art is replete with a number of patents and articles which describe the effect. As will be recognized by a person skilled in the art, the electrophoretic effect operates on the principle that certain particles, when suspended in a medium, can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. Electrophoretic image displays (EPIDs) utilize the electrophoretic effect to produce desired images. In prior art EPIDs, colored dielectric particles are suspended in a fluid medium that is either clear or an optically contrasting color as compared to the dielectric particles. The colored electrophoretic particles are then caused to selectively migrate to, and impinge upon, a transparent screen, thereby displacing the fluid medium against the screen and creating the desired image.
As will be recognized by a person skilled in the art, the selection of the electrophoretic particles used in the EPID is very important in determining the performance of the EPID and the quality of the viewed image produced. Ideally, electrophoretic particles should all be of a uniform size, to help in assuring that each of the electrophoretic particles will behave similarly. Additionally, it is desirable to utilize electrophoretic particles that have essentially the same density as the fluid medium in which they are suspended. By using electrophoretic particles of essentially the same density as the suspension medium, the migration of the electrophoretic particles through the medium remains independent of both the orientation of the EPID and the forces of gravity.
To effect the greatest optical contrast between electrophoretic particles and the suspension medium, it is desirable to have either white particles suspended in a black medium or black particles suspended in a backlighted clear medium. In the prior art, it has been proven difficult to produce black electrophoretic particles that are dielectric, of uniform size and have a density matching that of a common suspension medium. As a result, EPIDs commonly use readily manufactured light colored electrophoretic particles suspended in dark media. Such EPIDs are exemplified in U.S. Pat. No.: 4,655,897 to DiSanto et al., U.S. Pat. No. 4,093,534 to Carter et al., U.S. Pat. No. 4,298,448 to Muller et al., and U.S. Pat. No. 4,285,801 to Chaing. In such prior art, light colored particles are commonly inorganic pigments which have fairly high densities. With the electric field applied, the light colored particles migrate through the grayish suspension producing a light image on a gray background, thereby resulting in an image that is not highly contrasted.
Although titanium dioxide used in EPIDs produces a good optical contrast between the white particles and the suspension medium, it has a density about 4 g/cm.sup.3 which is too high to be matched with an organic solvent. Sedimentation becomes a problem. In the past decade, great effort has been spent to solve the density problem of titanium dioxide. Coating titanium dioxide particles with a polymeric material to reduce the density of titanium dioxide is an example. Phase separation, direct emulsification, emulsion polymerization and miniemulsion polymerization are the most common techniques used to make the polymer-coated titanium dioxide particles. The uniformity of the coating thickness and the size of final particles made by these methods cannot be well controlled, this causes a large difference in density among final particles, and balancing the density between the final particles and the suspension medium is still a difficult problem.
In application to EPID displays, the properties of the white particles is highly specialized. First, the density of the particles must be low and uniform in order to be balanced with the suspension medium to prevent sedimentation of the particles. Secondly, the particles must have well controlled surface functionalities for particle charging in dielectric media in order to reach an optimum electrophoretic mobility for electrophoretic imaging. Thirdly, the particles must have suitable crosslinking density as well as particle size and size distribution in order to form good whiteness yielding better optical contrast with the dark medium. Finally, the particles must have good heat and solvent resistance. Conventional techniques of making crosslinked polymer particles are suspension polymerization, emulsion polymerization, miniemulsion polymerization. Unfortunately, the properties of crosslinked polymer particles required in EPID are difficult to obtain by the conventional techniques.
Crosslinked polymer particles prepared by a suspension polymerization technique have wide particle size distribution, e.g. 1-50 .mu.m, which requires classification of the polymer particles. Polymer particles with a narrow particle size distribution can only be obtained at a very low yield. Emulsion polymerization can produce crosslinked polymer particles with a very narrow size distribution, however, it only can produce particle sizes in a sub-micron range (J. Appl. Phys., 26(7), 864 (1955)). In addition, only small amounts of crosslinking monomers can be used, producing particles with poor heat and solvent resistance and poor whiteness. Using the seeded emulsion polymerization technique, crosslinked polymer particles with uniform particle sizes greater than 1 .mu.m can be produced, however, it takes a long time to complete the whole process (Polym. Mater. Sci. Eng. 54, 587 (1986)). On the other hand, miniemulsion polymerization produces (J. Polym. Sci., Polym. Chem. Ed., 17,3069 (1979)) polymer particles having higher crosslinking density, however, the particle size distribution is too broad to obtain uniform electrophoretic mobility resulting in poor electrophoretic images.
It has been reported that it is difficult to produce stable uniform crosslinked polymer particles by a dispersion polymerization method when the crosslinking monomers is over 1% by weight (J. Polym. Sci., Polym. Chem. Ed., 24, 2995 (1986). Reports are also found for preparing styrene/divinylbenzene particles by batch dispersion co-polymerization and seeded dispersion copolymerization respectively (Colloid Polym. Sci., 269, 217 (1991), however, good monodispersity and heat and solvent resistance still can not be obtained. More recently, Kobayashi and Senna reported production of uniform styrene/divinylbenzene polymer particles with high crosslink density using a dispersion polymerization technique (J. Appl. Polym. Sci., 46,27 (1992)). Although they claimed that the highly crosslinked polymer particles are uniform in size and are greater than 1 .mu.m, no claim was made in regard to controlling of surface functionalities of the final particles, which is so important for particle charging in dielectric media, particularly in EPIDs.
In addition to using white particles suspended in a dark medium to produce contrast images, one may alternatively suspend black particles in a backlighted clear medium. However, as has been mentioned, the development of suitable dielectric black particles remains a goal in the art of electrophoretic image displays. In art other than EPIDs, black particles are commonly produced from carbon. However, carbon blacks are not readily adaptable to EPIDs because carbon blacks are conductive and the density of carbon blacks is not readily matched to a suitable suspension medium. Research efforts have been made in an attempt to solve the density and conductivity problems of carbon blacks, however, none has succeeded without trading off some blackness in the particles created. Such efforts to produce dielectric particles from carbon blacks are exemplified in the following article Hou et al. "Pigmented-Polymer Particles With Controlled Morphologies", (Polymer Latexes, ACS Symposium Series 492, Chap. 25, p. 405, 1992).